Melt-blown formed fabrics meet the ISO definition for formed fabric materials (ISO 9092:1988). According to it, a material is referred to as a formed fabric material if a) the fiber fraction is more than 50% by weight (except chemically broken down plant fibers) and the fibers have a coefficient of fineness greater than 300 or b) the following conditions are met: 1) the fiber fraction is more than 30% by weight (except chemically broken down plant fibers) and the fibers have a coefficient of fineness greater than 300 and 2) the density is less than 0.40 g/cm.sup.3.
This ISO regulation is also observed by the formed fabrics explained in further detail in the following, with these being produced according to the melt-blown process or a melt-blown technique. Without wishing to see this as a restriction, the melt-blown process can be described as follows: i.e. the melt-blown filaments, fibers and formed fabrics are generally produced as follows:
The particular synthetic material is placed into an extruder in which it is melted. From the extruder the melt is moved into the spinning head which comprises the melt-blown spinneret, which is the central component of the process. Here the melt is brought to the required processing temperature. The nozzle itself comprises a number of capillary bores. On both sides of the nozzle bores are disposed openings for the primary process air which is under pressure. Below the nozzle is a stacking arrangement in the form of a driven traveling screen or a revolving screen through which the fibers are drawn in and stacked to form a formed fabric.
As the melt exits from the nozzle bores, it comes into contact with relaxing hot primary process air at high speed. In the process the melt of each capillary bore is torn apart and drawn into a large number of fine fibers. In this process the filaments largely are torn to form fibers. This is in contrast to other spin formed fabric processes in which fiber breaks must be prevented. Through the primary process air stream cold ambient air, referred to as secondary air, is drawn in and conducted to the fibers and filaments being formed. The generated filaments and fibers are consequently cooled directly under the spinneret. The fibers are subsequently stacked on the above cited stacking arrangement to form a formed fabric and are wound. Melt bonding between the fibers, as a rule, does not take place. The fiber lengths are, as a rule, of the order of magnitude of 5 to 50 cm. The fiber diameter is very small and, for example in connection with the invention described in the following, is less than approximately 10 .mu.m.
Further information about the melt-blown process can be found in U.S. Pat. No. 3,825,379 (Exxon Research and Engineering Co.) as well as U.S. Pat. No. 4,714,647 (Kimberly Clark Corp.).
U.S. Pat. No. 4,869,275 also addresses the melt-blown process for the production of a formed fabric from various starting materials. As suitable starting materials are cited polyolefins (polypropylene, polyethylene and ethylene/propylene copolymers), polystyrene, polyester (polyethylene terephthalate), Nylon (6, 66 and 610), polymethylene methacrylates and generally also cellulose acetate. This patent does not specify the degree of substitution of this cellulose acetate when used in the described process. The unusual reference that even cellulose acetate is suitable ("even cellulose acetate" s. column 5, paragraph 1) indicates that it is only conditionally suitable. This is also in agreement with the technical findings that the narrow temperature interval between melting temperature and decomposition range largely excludes the conversion of the cellulose ester into processable melts, for example in the case of cellulose triacetate, and, in the case of lower melting cellulose acetopropionates, is still connected to incipient product damage (cf. Kunststoff-handbuch 3/1 Hansa Verlag, 1992, p. 411). If, in fact, cellulose acetate were processed into a melt-blown formed fabric at a high "melt temperature" which must be assumed, an undesirable strong degradation would occur. The degradation products would have a strong disadvantageous effect in various applications, thus in particular also when used as filter materials in tobacco smoke filters. Precisely this application is emphasized in U.S. Pat. No. 4,869,275. However, in the description of the especially practical embodiments, cellulose acetate is not taken into consideration. Due to the decomposition of cellulose acetate, which must be anticipated according to the known process, the quality of the obtained melt-blown formed fabric would also be impaired because no satisfactory degree of whiteness develops. In view of the decomposition of cellulose acetates at relatively high temperature, it should be pointed out that, beginning at 180.degree. C., a marked chemical decomposition occurs which can be detected inter alia through the formation of furfural.
According to Example 5 of U.S. Pat. No. 3,509,009 a portion of the cellulose acetate and a portion of diethylphthalate (as softening agent) are melt-spun at a temperature of 170.degree. C., so that decomposition of the cellulose ester used is largely excluded, but the product properties are dominated in an undesirable way by the softening agent. Such high content of softening agent restricts the application properties to the effect that too low a melting point is set as well as softening agent migration or exudation and exhalation can occur.